Porous polymers having numerous fine interconnected pores are used in various fields, including separation membranes used for applications such as the manufacture of ultrapure water, the purification of chemicals, and water treatment; breathable waterproof films used for products such as clothing and sanitary materials; and separators used for batteries such as secondary batteries.
Secondary batteries are widely used as power supplies for OA, FA, household electrical devices, and portable devices such as communication devices. In particular, portable devices that use lithium-ion secondary batteries are becoming widespread because these batteries provide high volume efficiency when mounted on devices and thus contribute to a reduction in device size and weight. Large secondary batteries, on the other hand, have been researched and developed in many fields related to energy and environmental issues, including load leveling, UPS, and electric vehicles and lithium-ion secondary batteries, which are a type of nonaqueous electrolyte secondary battery, have been used in a wide range of applications because these batteries provide high capacity, high power, high voltage, and high long-term storage stability.
The operating voltage of lithium-ion secondary batteries is typically up to 4.1 to 4.2 V. For such high voltages, aqueous solutions cannot be used as electrolyte solutions because electrolysis occurs. Accordingly, electrolyte solutions that use organic solvents, i.e., nonaqueous electrolyte solutions, have been used as electrolyte solutions resistant to high voltages. Solvents used for nonaqueous electrolyte solutions include high-dielectric-constant organic solvents, which allow more lithium ions to be present therein, and typical high-dielectric-constant organic solvents are organic carbonate ester compounds such as propylene carbonate and ethylene carbonate. Also, a highly reactive electrolyte, such as lithium hexafluorophosphate, dissolved in a solvent is used as a support electrolyte, which serves as a lithium ion source.
To prevent an internal short circuit, a lithium-ion secondary battery includes a separator disposed between a positive electrode and a negative electrode. The separator requires insulating properties because of its role. The separator also requires air permeability to provide channels for lithium ions and a fine porous structure to provide the function of diffusing and retaining the electrolyte solution. To meet these requirements, a porous film is used as the separator.
Battery safety has become increasingly important with the increasing battery capacity in recent years. Among the characteristics that contribute to the safety of battery separators are shutdown characteristics (hereinafter referred to as “SD characteristics”). SD characteristics refer to the function of closing fine pores in the porous film at high temperatures, i.e., about 100° C. to 150° C., to shut off ion conduction inside the battery, thereby preventing a further rise in the internal temperature of the battery. The lowest temperature at which the fine pores in the porous film are closed is referred to as shutdown temperature (hereinafter referred to as “SD temperature”). Porous films require SD characteristics when used as battery separators.
However, with the increasing energy density and capacity of lithium-ion secondary batteries in recent years, accidents have occurred in which, because the normal shutdown function does not work sufficiently, the internal battery temperature rises above about 130° C., which is the melting point of polyethylene, a material for battery separators, and failure of the separator due to thermal shrinkage causes a short circuit between the two electrodes and eventually leads to a fire. Accordingly, to ensure safety, there is a need for a separator having a higher heat resistance than those having the current SD characteristics.
To meet the need, multilayer porous films have been proposed that include a polyolefin-based resin porous film and a porous layer containing inorganic fine particles such as metal oxide particles and a resin binder on at least one surface of the polyolefin-based resin porous film (Patent Documents 1 to 3). These porous films are reported to provide extremely high safety because a coating layer heavily filled with inorganic fine particles such as α-alumina prevents a short circuit between the two electrodes even if the temperature continues to rise above the SD temperature in the event of abnormal heat generation.